This invention was supported in part by grant number 93J006 from Japan Science and Technology Corporation (JST). The Japan Science and Technology Corporation has certain rights in the invention.
The present invention relates to the field of generation of quantum-mechanical states of light. In particular, it relates to the development of devices to produce a stream of regulated and directed single pairs of photons.
Recent progress in the field of quantum optics has enabled scientists to perform experiments that test the fundamental principles of quantum mechanics, which were previously only possible as thought experiments. Furthermore, scientists have come to realize that those fundamental principles can be exploited technologically. For example, there is growing interest in the new fields of quantum cryptography, quantum teleportation and quantum computing. These experiments require that a quantum system be prepared in a well-defined state. Out of the many candidate systems, single photons or pairs of photons have been most widely used.
The recently demonstrated scheme of quantum cryptography involves encoding information on the polarization of a single photon or pairs of polarization-entangled photons. Protection against eavesdropping is provided by the quantum-mechanical fact that measurement of the information will inevitably modify the state of the photon. The single-photon version of quantum cryptography (BB84 protocol) is vulnerable if more than one photon is sent by mistake, and therefore a stream of regulated single photon is needed. The entangled-photon-pair-version (Ekert protocol) does not have this vulnerability, but nevertheless, a compact source of regulated pairs of polarization-entangled photons would make this scheme more attractive as a method of rapid and secure communication.
Other technological applications are possible for a device that can generate regulatediphoton streams. For example, the regulated photon stream will have very stable intensity, with fluctuations well below those of standard light sources. The device could thus potentially see use as a high-precision light standard. It could also be applied in classical low-power optical communications networks. A stream of single photons at regular time intervals provides a rapid stream of bits, which can potentially be used to store information. This represents the lowest possible power consumption in optical communication: one photon per bit.
The principle sources of single photons in use are highly attenuated lasers, or light-emitting diodes (LED""s). Optical pulses from the sources are reduced in intensity by absorption or reflection until each pulse contains, on average, less than one photon. Since the deletion of photons during attenuation is a random process, the number of photons in each pulse is also random. Many pulses will contain no photons, and some will contain more than one. This will limit the rate at which measurements can be done, and will also lead to errors in the experiments. It would be preferable to have a regulated source, where the number of photons in each pulse is well known.
Progress toward this goal was achieved by using single atoms, single trapped ions, or single molecules. An atom, or ion, or molecule was excited using a laser beam, and the resulting emitted light was observed. Because there is a certain amount of dead time between emissions of photons, the output photon flux is better regulated than for a laser or LED. In fact, if the exciting laser was pulsed in the right way, it would in principle be possible to obtain exactly one output photon per input pulse. However, these methods of producing regulated photons require complex and delicate experimental setups, and are thus not easily reproduced or used. Another difficulty in these methods is that the direction of photon emission is random. In other words, the photons fly in all directions, and are thus not easily collected and used in a subsequent experiment or system.
Another proposed source that overcomes the problem of random emission direction involves the use of strongly interacting photons in a nonlinear cavity. An optical cavity is used to enclose an atomic medium, which is exposed to a coupling laser beam that allows for strong non-linearity in the absence of loss. A pulsed laser beam is directed towards one end of the cavity. If the pulses have the correct shape, the output pulses from the other end of the cavity will each contain one, and only one, photon. This stream of regulated single photons will be directed in a well-known direction. However, the experiment setup is again quite complex and difficult to operate. It is thus difficult to incorporate into a large experiment or to use in a technological application.
One more proposed source involves pumping a quantum dot with a surface acoustic wave (SAW) (xe2x80x9cPhoton Trains and Lasing: The Periodically Pumped Quantum Dotxe2x80x9d by C. Wiele et al., published in October 1998 in Physical Review A, vol. 58). The quantum dot is a small region of semiconductor material that can contain only one electron and one hole. The SAW is a periodic deformation that travels along the semiconductor surface. The wave can trap electrons and holes and move them along the surface. It may be possible to make the wave such that only one electron and one hole will be transported in each period. If the wave then passes over an appropriate quantum dot, the electron and hole will be trapped by the dot. They will then recombine to produce a photon. The photons will not be emitted in any particular direction. As well, it is not yet evident whether it will actually be possible to create a SAW such that each period contains exactly one electron and one hole. Finally, there will be errors in the output photon stream when the dot fails to trap both carriers.
A single photon turnstile device was realized in a mesoscopic double barrier p-i-n junction (xe2x80x9cA Single-Photon Turnstile Devicexe2x80x9d by J. Kim et al., published in Feb. 11, 1999 in Letters to Nature, vol. 397, and xe2x80x9cTurnstile Device for Heralded Single Photons: Coulomb Blockade of Electron and Hole Tunneling in Quantum Confined p-i-n Heterojunctionsxe2x80x9d by A. Imamoglu, published in Jan. 10, 1994 in Physical Review Letters, vol. 72). Regulated single photons were produced using a combination of simultaneous Coulomb blockade effect for electrons and holes and resonant tunneling in a mesoscopic p-n junction. The structure generally comprises of an intrinsic central quantum well (QW) in the middle of a p-n junction, and n-type and p-type side quantum wells (QWs) isolated from the central QW by tunnel barriers (FIG. 1a). The lateral size of the device is reduced to increase the single-particle charging energy e2/2Ci where Ci (i=n or p) is the capacitance between the central QW and the side QWs. The device is designed such that the electron and hole tunneling conditions are separated in applied bias voltage, and thus can be controlled independently. The electron resonant tunneling condition into an electron sub-band in the central QW is satisfied at a certain bias voltage V0. When an electron tunnels, the Coulomb blockade effect shifts the electron sub-band energy off of resonance, so that the subsequent electron tunneling is inhibited (FIG. 1b). Then the bias is increased to V0+xcex94V to satisfy the hole resonant tunneling condition. If a single hole tunnels into the hole sub-band of the central QW, the subsequent hole tunneling is inhibited due to the Coulomb blockade effect for holes. By modulating the bias voltage between the electron and the hole resonant tunneling conditions periodically, it is possible to inject a single electron and a single hole into the central QW periodically, if the tunneling time is much shorter than the pulse duration. If the radiative recombination time of an electron-hole pair is also much shorter than the pulse duration, one (and only one) photon is emitted per modulation period.
A GaAs/AlGaAs three-QW structure sandwiched by n-type and p-type AlGaAs bulk layers was grown by molecular beam epitaxy.
Post structures with diameters of 200 nm-1.0 xcexcm were made by electron-beam lithography followed by metal evaporation, lift-off, and BCl3/Cl ECR plasma etching. The surface of the device was passivated with sulfur in (NH4)2S solution, and encapsulated with a silicon nitride film. Finally, the structure was planarized with hard-baked photoresist, and bonding pads were evaporated. The top semi-transparent metal served as the p-type contact from which an emitted photon was detected, and the n-type contact was formed in the substrate. The device was installed in a dilution refrigerator with a base temperature of 50 mK and biased with DC and square wave AC voltages. The emitted photons were detected by a silicon solid-state detector. FIG. 2a shows a histogram of the measured time intervals between the rising edge of the driving pulse and the photon detection. The data show that the emitted photons follow the rising edge of the driving pulse, as expected.
Unfortunately, the turnstile device described above exhibits low detection efficiency of generated photons due to the low escape probability of the photons from the structure. The substrate is opaque for the emitted light and photons must pass through opaque metal contacts. Consequently, only a small fraction of the generated photons (2xc3x9710xe2x88x923) reach the photodetector.
Furthermore, a considerable background leakage current produces non-regulated photons. The leakage current is due mainly to the fact that the electron tunneling cannot be fully suppressed at a high bias voltage when only holes should be allowed to tunnel. This is because the peak-to-valley ratio of the resonant tunneling structure was finite. Due to the first two items, the photon states that are generated by the device are a stream of single photons with their time intervals only slightly more regular than those of a random Poissonian source.
Prior art photon turnstile devices also exhibit low modulation frequency. As can be derived from FIG. 2, the photon recombination time in the present device was 30 nsec. This limits the modulation speed to below 10 MHz. In addition prior art devices typically require low operation temperatures. In a turnstile device, which relies on Coulomb blockade, the thermal energy fluctuations must not exceed the Coulomb blockade energy shift. This energy shift is determined by the device capacitance and therefore requires. the very low temperatures in the experiment described above.
Development of sources of polarization-entangled photon pairs has occurred slowly over the last few decades. The atomic cascade method, developed in the early 1980""s, uses a special two-photon decay process in atoms such as calcium. Although the photons are emitted in all directions, polarization entanglement is perfect only when two photons from a pair are emitted in the same or opposite directions. Spontaneous parametric down conversion, developed in the mid-1980""s, uses a nonlinear crystal to convert pump photons into entangled photon pairs. Although this method can generate entangled photon pairs at a decent rate, the number of pairs produced cannot be precisely controlled, but rather follows a Poisson distribution.
There is a need, therefore, for a quantum-dot photon turnstile device that overcomes the above difficulties.
It is an object of the present invention to generate pairs of polarization-correlated or polarization-entangled photons. It is another object of the invention to provide a quantum-dot photon turnstile device having a high probability of escape for generated photons. It is a further object of the invention to provide a quantum-dot turnstile device that exhibits low production of non-regulated photons. It is another object of the invention to provide a quantum-dot turnstile device that exhibits high modulation frequency operation. It is an additional object to provide a photon turnstile device capable of high temperature operation.
A quantum-dot photon turnstile device includes a quantum dot embedded in a resonant tunneling barriers, and an external optical cavity coupled to the quantum dot. The resonant tunneling structure includes an intrinsic semiconductor disposed between a heavily-doped p-type semiconductor layer and a heavily-doped n-type semiconductor layer. The quantum dot is embedded in the intrinsic semiconductor layer. The dot has an average base width of roughly 20 nanometers and a height of roughly 4 nanometers. The intrinsic material surrounding the quantum dot forms tunnel barriers, which separate the electron reservoir, i.e., the n-type semiconductor layer, and the hole reservoir, i.e., the p-type semiconductor layer, from the quantum dot.
According to an exemplary embodiment, the device is capable of producing a regulated and directed stream of single pairs of photons with opposite circular polarizations. The device is first biased at an electron bias voltage Ve such that two electrons with opposite, spins can tunnel into the initially empty quantum dot. Further electron tunneling is now suppressed due to the Pauli exclusion principle, since the ground state is filled and the next available electron state, the first excited state, is far off of resonance. Then the device is bias at a hole bias voltage Vh such that two holes with opposite spins can tunnel into the dot. Again, further hole tunneling is suppressed due to Pauli exclusion principle since the hole ground state is filled and the first excited hole state is off of resonance. Once two holes have tunneled, two electrons recombine with two holes as dictated by selection rules, producing a pair of photons with opposite circular polarizations. From here, the cycle is repeated. Thus, modulating the bias voltage between Ve and Vh produces a regulated stream of photons, where two photons are emitted per modulation cycle. The emission frequency of the single pairs of photons can be changed by adjusting the voltage modulation frequency.
For polarization anticorrelation to be observed, the spin-relaxation times for electrons and holes in the dot must be longer than the electron-hole recombination time (true for typical semiconductors), and the recombination time must be much longer than the hole tunneling time. Therefore, the electron bias voltage Ve and hole bias voltage Vh are selected to maximize the tunneling probabilities of two electrons and two holes into the quantum dot. Furthermore, the doping level of the n-type and p-type semiconductor layer and the thickness of the intrinsic tunnel barriers are controlled to ensure that the hole tunneling time is longer than the recombination time. If the spin-dephasing rate for electrons and holes is slow enough, then the emitted photons will also have entangled polarizations.
The optical cavity directs the emitted photons into a single electromagnetic mode. Without such a cavity, the spontaneously-emitted photons would leave randomly in all directions. The cavity typically comprises a pair of reflectors placed on opposite sides of the quantum-dot structure, such as distributed-Bragg-reflector cavity or Fabry-Perot cavity. Alternatively, the cavity can be a microsphere cavity, a simple post with or without a metal coating, or a photonic bandgap structure.